This invention relates to a method and apparatus for monitoring a hydrogen containing gas stream.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer membrane-electrolyte having the anode on one of its faces and the cathode on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cells gaseous reactants over the surfaces of the respective anode and cathode catalysts. A plurality of individual cells are commonly bundled together to form a PEM fuel cell stack. The term xe2x80x9cfuel cellxe2x80x9d is often used to refer to an individual cell and also may refer to a fuel cell stack which contains many individual fuel cells often on the order of one hundred or more, connected in series. Each cell within the stack includes the membrane electrode assembly (MEA), and each such MEA provides its increment of voltage. A group of cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, in admix with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies (MEAs) which comprise the catalyzed electrodes are relatively expensive to manufacture and require certain controlled conditions in order to prevent degradation thereof.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished heterogeneously within a chemical fuel processor, known as a reformer, that provides thermal energy throughout a catalyst mass and yields a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,442 and 08/980,087, filed in November, 1997, now U.S. Pat. Nos. 6,232,005 issued on May 15, 2001 and 6,077,620 issued on Jun. 20, 2000, respectively. and U.S. Ser. No. 09/187,125, filed in November, 1998, now U.S. Pat. No 6,238,815 issued on May 29, 2001, and each assigned to General Motors Corporation, assignee of the present invention. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
The reforming reaction is an endothermic reaction that requires external heat for the reaction to occur. The heat required to produce enough hydrogen varies with the demand put on the fuel cell system at any given point in time. Accordingly, the heating means for the fuel processor must be capable of operating over a wide range of heat outputs. Heating the fuel processor with heat generated externally from either a flame combustor or a catalytic combustor is known. U.S. patent applications Ser. Nos. 08/975,422 and 08/980,087 filed in the name of William Pettit in November, 1997, now U.S. Pat. Nos. 6,232,005 issued on May 15, 2001 and 6,077,620 issued on Jun. 20, 2000, respectively, and assigned to the assignee of the present invention, disclose an improved catalytic combustor, and the integration thereof with a fuel cell system which fuels the combustor with unreformed liquid fuel, hydrogen-containing anode exhaust gas from the fuel cell, or both. The operating cycle depends on many factors, such as anode stoichiometry, steam/carbon ratio, electrical demand placed on the system, etc.
Thus, load changes placed on the fuel cell resulting in greater or lower power output requirements, requires the fuel processor to generate more or less hydrogen. Correspondingly, since the combustor generates whatever heat input is required to sustain the chemical reactions within the fuel processor, the combustor likewise must generate more or less heat to maintain the required reaction temperatures within the fuel processor. The control of heat production by the combustor is dependent upon several parameters, one of the principle ones being the fuel flow to the combustor, and particularly anode exhaust gas from the fuel cell.
A vehicular fuel cell system requires a fast response to fuel cell load changes. In some situations, the combustor may not be able to accept all of the anode exhaust gas being supplied. Prior control devices used to control hydrogen-containing gas which is not consumed by the anode demonstrate slow response times. Therefore, a problem results from the use of anode hydrogen-containing effluent gas as a fuel source to the combustor. Since the combustor is fueled by different sources, and in different modes, i.e., start-up, warm-up, running mode, conventional sensors which monitor overall anode effluent volume or mass flow do not account for the actual mass flow rate of hydrogen. Another problem is that actual mass flow rate of hydrogen to the fuel cell stack is difficult to accurately monitor on a real-time basis. The demand for hydrogen by the stack changes in response to fuel cell load changes which are often very rapid. Thus, it would be desirable to provide a hydrogen flow control method and apparatus which gives an accurate indication of hydrogen mass flow rate. It is also desirable to have such method and apparatus which has a fast response.
A further problem posed by fuel cell systems is the degradation of precious metal catalytic components of the electrode layers of the MEA. The catalytic sites become poisoned or occupied by carbon monoxide. Thus, reactive surface is lost due to carbon monoxide poisoning, and less reactive surface is available to catalyze fuel cell reaction of hydrogen and oxygen. Thus, it would be desirable to provide a method and apparatus to monitor the effect of carbon monoxide poisoning, and to detect progression of such poisoning before an excessive amount of catalytic reactive surface is rendered ineffective.
The present invention is an apparatus and method for monitoring a hydrogen-containing gas stream and optionally a non-hydrogen gas including carbon monoxide.
The apparatus includes a sensor assembly comprising of at least two electrochemical cells or membrane electrode assemblies (MEAs) electrically isolated from one another and sequentially arranged in the path of a gas stream containing hydrogen. Each MEA includes a reactive surface area wherein hydrogen in the gas stream is sequentially passed over the reactive surface, is consumed, and a current is generated. The voltage of at least the first two sequential MEAs is regulated. Preferably, the potential of these MEAs is the same. The combined current generated by all of the sequential MEAs is proportional to the quantity of hydrogen consumed by the sensor and in the gas flow path entering the sensor.
The sensor assembly may used in combination with a laminar flow conduit whereby a portion of the hydrogen containing stream is diverted to the sensor and a current is generated.
The sensor may also be used in combination with a flow meter which measures the flow rate of the bulk gas stream in the laminar flow conduit from which the sensor receives a diverted stream therefrom. The amount of hydrogen in the diverted stream is proportional to the amount of hydrogen in the bulk gas stream in the laminar flow conduit.
The apparatus may also be used to monitor a non-hydrogen gas, such as carbon monoxide, by comparing the current generated by the individual MEAs.